Modern railway passenger cars are usually fabricated from "angle" (L), "zed" (Z) or "top hat" ( ) metal stock welded together to constitute a frame which is subsequently clad with sheet metal, such as stainless steel sheeting, spot welded or otherwise affixed to the frame.
It is difficult, however, to maintain the sheet flat during the cladding process, so that the external appearance of a railcar so clad may present an uneven appearance, or what is termed an "oil can" effect; that is to say, a rippled or dimpled appearance. Moreover, spot weld markings and heat discolorations also tend to mar the external appearance.
While such surface imperfections have purely a visual impact and are of no structural significance, reflections from the metal surface tend to exaggerate the unevenness of such cladding sheets and are quite noticeable when the cars are in service. This may well be unacceptable to the purchaser who may equate such with fragility or insubstantial construction.
Several methods have been proposed to mitigate surface imperfections, including welding stiffening members to the rear face of the sheet, finely grooving the surface, and even employing deeply corrugated or fluted panels. In conventional present day cars, such corrugated panels are widely used to conceal spot weld marks and heat discolourations associated therewith. However, when the use of large flat panels are mandatory, it has been proposed to fabricate a composite panel comprised of a "honeycomb" aluminium core "sandwiched" between stainless steel sheet stock, ideally adhesively bonded thereto. Such a panel is described and claimed in the specification relating to Australian Patent Application No. 66401/88. While this is considered to be an acceptable solution to the abovementioned problems, further investigations have led to the improved composite metal panels of the invention.